The advantage of anti-rat thymocyte globulin treated newborn Wistar rats over the nude mouse system in revealing the metastatic potential of continuous cell lines was demonstrated previously. We were able to significantly in"crease the sensitivity of the rat system for low or moderately malignant continuous cell lines by using the intraperitoneal route for inoculation of cells and by increasing the number of cells per inoculum. An abstract will be published in August 1990 and a poster presentation is accepted by the 15th International Cancer Congress.